1. Field of the Invention
The present invention relates to gas discharge lasers, in particular to excimer lasers, and more particularly to a discharge unit for an excimer laser, wherein a high voltage is supplied to two discharge electrodes.
2. Background of the Invention
Excimer lasers provide high intensity laser radiation in the ultraviolet spectral range. This makes them important tools especially for medical and surgical applications as well as other industrial applications.
Excimer lasers are gas discharge lasers that use a rare gas such as argon and a halide gas such as fluor (for example AF excimer laser) or a gas containing a halide (for example F2) as the laser gas.
Generally, in an excimer laser a gas mixture containing the active component and other gases is steadily provided to a discharge gap between a pair of elongated electrodes inside the laser tube by means of a fan or the like. A high voltage applied between the two electrodes causes a gas discharge in said discharge gap, whereby, from the active component, short-lived excited-state molecules are generated, whose disassociation gene-rates ultraviolet radiation constituting the laser radiation. To increase the homogeneity of the gas discharge, in present excimer lasers a pre-ionization of the laser gas by pre-ionizers is used. As the used laser gas needs to regenerate before it can be reused, excimer lasers are generally operated in a pulsed operation mode, wherein the fan is steadily replacing the laser gas in the discharge gap.
The discharge electrodes of an excimer laser are usually located inside the laser tube.
The housing of an excimer laser generally consists of a metal tube having openings in a cylindrical wall on the upper side thereof. An insulating plate covers the open upper side. The metal tube and one of the discharge electrodes are grounded. A high voltage is applied to the second discharge electrode via a HV duct extending through the insulating plate.
The width of the discharge gap between the two electrodes has to be adjusted very accurately, so as to achieve a homogeneous gas discharge in the discharge gap.
The optical axis of the laser beam is formed by the arrangement of the two opposite electrodes. The electrodes in the laser tube have to be adjusted lengthwise and perpendicular to the optical axis. This has to be done very accurately to avoid arcing between the electrodes and to avoid hot spots in the excimer laser beam profile.
Present excimer lasers are constructed by first providing a laser tube, and then most of the parts constituting the laser are mounted into the tube one by one. In particular, usually the laser tube consists of two parts, e.g. a main part and a cover. One discharge electrode is mounted to the main part of the laser tube, and the other discharge electrode is mounted to the cover. After this, the main part and the cover are connected, so as to close the laser tube. As a result, the discharge electrodes are assembled, so as to generate the discharge gap between them. The accuracy of the discharge gap is determined by the accuracy of the joint between the main part and the cover of the laser tube. This makes a correct adjustment of the discharge electrodes with respect to each other difficult.
Such an assembly is explained for example in U.S. Pat. No. 5,771,258.
In other more flexible arrangements, the electrodes can be adjusted to each other in the pre-mounted tube. This is usually done by a gauge that is placed in between the electrodes from both sides of the pre-mounted laser tube.